1. Field of the Invention
The present invention relates to a non-radiative dielectric waveguide circuit which can generate multiple frequencies, compared to a conventional non-radiative dielectric waveguide circuit which can generate only a single frequency.
2. Description of Related Art
In the age of multimedia, the content of information delivery has changed from text to text and graphics, and now it is changing to also include moving images. Moreover, the technique of utilizing moving images is now being improved to provide high resolution and delicate color. As a result, computer communication has undertaken the heavy burden of multimedia information delivery.
As one way to overcome this problem, computer communication is being upgraded from T1 level to T3 level. However, this upgrade has some shortcomings; for example, it is a costloy mode of communication because the cost of optical fiber construction is enormous. Furthermore, sometimes the construction itself gets to be difficult in places where a building has already been built. As a result, we are facing the need to develop an ultra-highspeed radio transmitter that corresponds to the demand for ultra-highspeed terminals for radio data communication.
In order to increase the speed of data transmission, we need to raise a using frequency. Raising the using frequency, however, causes a wavelength to become shorter. One important problem with this is propagation loss, We can generally think of the MMIC method as a way of dealing with more than the microwave frequency band. At the frequency of 50 GHz, however, this method shows the propagation loss of 60 db per 1 m.
To solve the problem of propagation loss, the method of utilizing a non-radiative dielectric waveguide has been introduced. The method is considered to be preferable with some advantages. Since the method has a non-radiative characteristic, its propagation loss is a lot smaller than that of a microstrip circuit. In addition, it""s much easier to build a transmission line with a non-radiative dielectric waveguide than with a waveguide. In particular, the non-radiative dielectric waveguide possesses the capability of dealing with a millimeter wave band of more than 30 GHz.
The non-radiative dielectric waveguide prevents transmission waves from leaking out of the waveguide. This is achieved by inserting the non-radiative dielectric waveguide(2) between two parallel metal plates(1) as in FIG. 1. When transmission waves flow along the non-radiative dielectric waveguide, either LSM mode or LSE mode is generated as shown in FIG. 2. In order to minimize propagation loss and fabricate a circuit in a convenient way, we use LSM 01 mode, which is the lowest mode of LSM mode.
The size of a non-radiative dielectric waveguide is defined by the equations below:                                                                         ϵ                r                            -              1                                ⁢                      b            /            λ                          ≈                  0.4          ~          0.6                                    Eq.  2            
where ∈r is the dielectric constant and xcex is the wavelength.
As an example of the above equations 1 and 2, suppose that a non-radiative dielectric waveguide has a dielectric constant of 2.04 and a wavelength of 5 mm at a frequency of 60 GHz. Then, solving for a and b, the waveguide has a height a of 2.25 mm and a width b of 2.5 mm. In other words, the space between the metal plates turns out to be 2.25 mm.
When a circuit is fabricated, it is necessary to place a spacer made of metal between the two metal plates as shown in FIG. 3. For the above experiment, the spacer should have a height of 2.25 mm. The spacer is supposed to be inserted into a non-radiative dielectric waveguide circuit so as to maintain a specified space between the two parallel metal plates. Hence, all other elements, including a non-radiative dielectric waveguide, should be the same height as the spacer. Since the space between the two metal plates is kept constant in the way described above, it is impossible to insert different-sized parts or a different-sized non-radiative dielectric waveguide between the plates. Considering that the size of a non-radiative dielectric waveguide is determined by the wavelength of a frequency, it can be inferred that we are able to propagate only a single frequency because the space between the metal plates is fixed. Therefore, the above approach cannot be applied to most of existing high frequency circuits which use multiple frequencies.
We generally make use of the method of super-heterodyne for most high frequency circuits such as an AM/FM receiver, television set, cellular phone, pager, cordless telephone, walkie-talkie or satellite receiver. The super-heterodyne method is helpful in developing a highly sensitive receiver because it converts a frequency in the middle of signal-processing to enhance amplitude. Also in the case of a transmitter, in order to promote its modulation characteristic, the amplitude of a frequency gets increased by modulating at a low frequency and multiplying it, and those operations finally cause to increase its modulation index. By contrast, a non-radiative dielectric waveguide circuit, which has a fixed gap between two plates cannot make any change in frequency, using only a single frequency. Consequently, it is impossible to construct such ultra-high frequency circuits mentioned above by use of the conventional non-radiative dielectric waveguide circuit.
Furthermore, the data transmission speed is now becoming faster. Accordingly, a using frequency is getting higher, and that requires the size of a non-radiative dielectric waveguide to be smaller. As an example, if a frequency is 60 GHz, the size of a non-radiative dielectric waveguide is 2.25 mmxc3x972.5 mm. If a frequency is 120 GHz, however, the size of a non-radiative dielectric waveguide is 1.125 mmxc3x971.25 mm that is half the size of the non-radiative dielectric waveguide in the case of 60 GHz. For the frequency of 120 GHz, even if the waveguide is invented, semiconductors such as a gunn diode is unable to be inserted into a space of 1.25 mm. This is because a semiconductor with a diameter of 3 mm is usually sold and it is hard to downsize due to inner calorification.
An object of the invention is to provide a non-radiative dielectric waveguide circuit with multiple layers on each metal plate and with multiple non-radiative dielectric waveguide in different sizes being available to multiple different frequencies.